Mechanism of cell polarization and asymmetric segregation of ageing determinants Abstract: Asymmetric cell division is a fundamental mechanism employed in diverse organisms to segregate aging determinants, diversify cell fate, and form distinct patterns of cell function in tissues and organs. The established of cell polarity is a critical initial event in asymmetric cell division, as cell polarity directs the partitioning of cellular components and determines the orientation of the spindle and cell division axes1. Recent studies in the budding yeast Saccharomyces cerevisiae, which undergoes asymmetric cell division during its vegetative growth, have provided important insights into the mechanisms and design principles underlying cell polarity and asymmetric cell division. The goal of this project is to elucidate the mechanism of symmetry breaking during the establishment of cell polarity and understand how the axis of cell polarity allows segregation of aging determinants for the continuous renewal of a "youthful" population. Three specific aims are proposed. The first two aims center on the mechanism of cell polarization, investigating a poorly understood mechanism of cell polarization that does not involve the actin cytoskeleton. We plan to test the hypothesis that autocatalytic targeting of Cdc42 GTPase, one of the evolutionarily conserved, master regulators of cell polarity, and delayed activation of Cdc42 GTPase activating proteins (GAPs) are crucial mechanisms of the actin-independent symmetry breaking. The third aim focuses on how cell polarity directs asymmetric distribution of new and aged multi-drug resistance (MDR) proteins. We have recently demonstrated that a group of MDR proteins are important ageing determinants asymmetrically segregated between the mother and bud during yeast asymmetric divisions. The proposed study will test the possible role of polarized protein translation in the asymmetric deposition of newly synthesized MDR proteins and will use an unbiased genome-wide screen to identify the molecular pathways that regulate MDR protein asymmetric inheritance. PUBLIC HEALTH RELEVANCE: The proposed project is relevant to public health because defects in cell polarity are associated with many major human health problems such as epithelial cancers, polycystic kidney disease, immune deficiencies and developmental defects. The ability to properly distribute ageing determinants along the axis of cell polarity is critical for the maintenance of stem cell replicative potential and the prevention of ageing-associated diseases. The proposed work will reveal fundamental mechanisms underlying the establishment and maintenance of cell polarity and asymmetric segregation of ageing determinants during cell divisions.